Which processes in space cause these mysterious auroral beads?

This post is written by Aurorasaurus guest blogger Nadine Kalmoni, a PhD student at Mullard Space Science Laboratory, University College London in the UK.

The first time I saw this incredible image of the aurora (Figure 1) was just before Christmas of 2015 as a twitter post by a member of the public. I remember thinking, “Wow!” Photos and videos of small scale auroral beads from this perspective with such incredible clarity and regularity are extremely rare, however similar beads have been observed multiple times by scientific imagers on the ground and through instruments on Earth-orbiting spacecrafts. We think these beads may be generated by instabilities in space or in Earth’s ionosphere. This blog post is intended to give an insight into the type of scientific analysis we do on auroral data to uncover the physics that causes these mysterious signatures.

Figure 1: Side-on photograph of auroral beads observed during a geomagnetic storm from Saskatoon, Canada using a DSLR camera. The beads have a 20 km spacing based on star-tracking and analysis.

This image surprises me not only because of its beauty and uniqueness, but also because I am a Ph.D. student specializing in studying regular, repeating patterns & structures within the aurora and deciphering their cause. Even though this type of aurora, auroral beads, is not an unusual to me, I had never seen them quite like this before.

The beads I’ve studied look more “beady” while beads in the twitter photo are better described as “stripey”. Auroral beads tend to look more or less beady or stripey based on the perspective from which the photos of the beads were taken. The twitter photograph captures the side profile of the beads, while scientific instruments often observe them directly from above or below. I was also surprised at how clearly the beads were captured in a single camera shot. The photographer, Alan Duffy of Saskatoon, Canada, managed to catch about 17 beads in that one shot. He studied physics and is an expert sky watcher who went out during a geomagnetic storm last December. He recognized that seeing this phenomena in the sky was rare and short-lived. It only lasted for a few minutes! This is pretty impressive because I have never seen more than 7 beads at any one time! So, I knew right away that this photo was very special.

Although phenomenal, the photograph settled into the back of my mind, as I continued other research. Not too long after that, in June of the following year, I met the Aurorasaurus team members Liz MacDonald, Aurorasaurus founder, and Burcu Kosar at a conference in Santa Fe, NM. The very same photograph had also caught their attention and they were able to perform some analysis on the image using the stars visible in the photo to calculate how far away the auroral arc was from the photographer. This helped them determine the distances between beads. They had even obtained videos of the same event from another citizen scientist connected to the Aurorasaurus project! Ashton Reimer, a Ph.D. student in space physics and an aurora enthusiast, was observing the same aurora, but in a different location. Ashton’s video showed the spacing of the beads too, but also contributed data the photograph couldn’t – the eccentric motion of the beads. It revealed that a group of beads propagate together in a single direction for a few seconds and then the motion suddenly reverses direction. Similar features have also been observed in other data of auroral beads obtained by the NASA’s THEMIS mission’s All-Sky Imagers. We don’t know why the beads sometimes suddenly change direction; it’s an open question we are currently investigating further.

Figure 2: Scientific ground-based observations of larger auroral beads during a substorm from Gillam, Canada using a THEMIS mission’s All-Sky Imager with a fish-eye lens and artificial colorscale. The separation between beads is roughly 65 km. The comparative 20 km scale size of the beads photographed by Alan Duffy are indicated.

Besides using DSLR cameras, auroral beads can also be observed using different scientific cameras. Figure 2 was captured using a ground-based camera called an All-Sky Imager. Imagine lying flat on the ground in an empty field looking up at a clear night sky to observe an auroral display, you would see the same as an All-Sky Imager would see. Your view of the sky is roughly circular, and may include some trees or houses populating the edge of the field you are lying in. The aurora would look very clear overhead, however the further you move towards the edges of your view, the less clear they would be. So in the All-Sky Imager data displayed in Figure 2, we can vaguely see the circular image and an auroral arc with very clear beading.

Figure 3: Space-based observation of auroral beads during a substorm observed from the IMAGE satellite using a Far UltraViolet camera (artificial colorscale). These beads are approximately separated by 300 km. The comparative 20 km scale size of the beads photographed by Alan Duffy are indicated.

Figure 3 is a picture of the auroral oval was taken from space by the IMAGE satellite.Towards the southern edge of the auroral oval, the auroral arc is much brighter and if you look closely, auroral beads can be identified. Looking at these images of auroral beads captured by 3 different methods, one wonders whether they are all caused by the same physical processes.

Since the beads appear in repeating patterns it is natural to think that they are caused by a wave. In space such waves are electromagnetic, meaning they can vary in both electric and magnetic quantities. The wavelength and the amplitude are parameters used to characterize a wave (see Figure 4a). The wavelength is the distance between two consecutive peaks (or troughs) in intensity. The wave represented by the purple line has a wavelength of 300 km while the wave represented by the green line has a wavelength of only 20 km. In our case, the wavelength corresponds to the distance between the centres of 2 neighbouring beads and the amplitude is equivalent to the brightness of the beads.

So, what can the bead spatial scale and amplitude tell us about the beads’ generation mechanism? By studying auroral beads in detail, we think that the different wavelengths of the auroral beads may mean that they are generated by different processes in different regions of the Earth’s magnetosphere.

Figure 4: (a) The wavelength and amplitude of 2 different types of auroral beads. The larger wavelength of 300 km (purple) is typical to those observed by the IMAGE satellite, and the smaller wavelength (green) is the same as observed by Alan Duffy at Saskatoon on December 21, 2015, (b) The Earth’s magnetic field topology is distorted due to the interaction with the solar wind. This gives rise to different plasma populations such as the plasma sheet, plasmasphere and radiation belts. We think that the large-scale beads are generated by the waves in the plasma sheet which propagate along the magnetic field lines, causing this beautiful auroral emission.

A wave that is generated out in the Earth’s magnetosphere travels along its magnetic field lines into the auroral regions of the upper atmosphere, allowing us to observe beautiful aurora from the ground and satellites. The source region of the beads with a smaller wavelength might be closer to the Earth, while the beads with a large wavelength could be generated much further out in the Earth’s magnetosphere. Whether the brightness of auroral beads increases, decreases or remains the same provides us with another clue for identifying the type of processes responsible for their generation. The beads shown in by the All-Sky Imager and satellite data in Figures 2 & 3grow very fast in brightness. This leads us to think that they may be generated by an instability in the near-Earth magnetosphere environment. Whether the smaller beads photographed by Duffy in Figure 1 are caused by an instability in the Earth’s ionosphere remains an open question!

Many of the images collected by the Aurorasaurus project through the citizen science campaign are truly extraordinary. The Duffy and Reimer photos allow scientists to study these signatures, which are not always caught by scientific instruments. Studying the various types of auroral beads helps us determine whether they are caused by similar disturbances. This research allows us to improve our understanding of the physical processes that happen within the Earth’s magnetosphere.

Acknowledgements:

Many thanks to Burcu Kosar for valuable discussions and input in helping me write this blog post.

I would also like to acknowledge V. Angelopoulos, S. Mende and E. Donovan for use of the THEMIS Mission’s all-sky imager data.

Figure 1: The photo was taken by Alan Duffy and the resulting image was analysed and adapted by Burcu Kosar.

Figure 3: The image was captured by the FUV/WIC imager on the IMAGE satellite, constructed by Mike Henderson and further analysed and discussed in Henderson, M. G. (2009), Observational evidence for an inside-out substorm onset scenario, Ann. Geophys.,27(5), 2129–2140, doi:10.5194/angeo-27-2129-2009.

Figure 4: (a) was created by Nadine Kalmoni. (b) was created by NASA and adapted by Nadine Kalmoni for the purpose of this article.

Very interesting! I have photographed the auroral beads twice in the last four months here in Washington. Last night was the second time. Another photographer posted an image of what looked like them originating from a proton arc. I would love to learn more. Are there any other articles available that you would recommend?